The invention relates to a method for subdividing cylindrical pieces of brittle non-metallic materials into wafer-form intermediate products. In this method, materials which have a Vicker hardness of up to HV 15000 N/mm.sup.2 can be subdivided. In particular, the materials which can be subdivided are the substrate materials for electronic structural parts, for example silicon, germanium or gallium arsenide. These materials make extreme demands on the machining process due to their special material properties.
The method in accordance with the invention is particularly important for subdividing semiconductor bars into semiconductor wafers which are highly planar on one side and have optimum surface quality.
The invention also relates to a device for subdividing cylindrical pieces of brittle non-metallic materials into wafer-form intermediate products.
The semiconductor raw materials for the semi-finished product are molded in cylinder form called the "bars" or "ingots". In order to perform additional working on the bar, a subdivision or slicing of the bars into thin discs, called "rounds" or "wafers", is required. The process of slicing the bars makes increasingly higher demands with respect to dimensional accuracy and surface quality of the wafers obtained in this way. To achieve these higher demands, the wafers should be plane-parallel as much as possible. Furthermore, it is of particular importance that one of the two faces of the wafer is as planar as possible so that it can serve as a reference plane for subsequent working on the bar.
It is known from West German Patent DE 36 13 2 C2, corresponding to U.S. Pat. No. 4,896,459, granted Jan. 30, 1990, and West German Patent DE 37 37 54 0 C1, corresponding to U.S. Pat. Nos. 4,967,461 granted Nov. 6, 1990 and 4,881,518 granted Nov. 21, 1989, to provide a device which suggests a solution to the requirement for a planar reference surface. These devices provide for an integration of the slicing and planing processes wherein the front face of the bar which is uneven due to the preceding slicing process, is planed through a milling removal process before a further wafer is sliced. After a further slicing process, the face, planed in this way serves in the wafer generated therein as planar reference surface for the further working.
As described in the above-cited publications, the planing process of the bar front face is carried out by grinding. The slicing process is preferably accomplished by means of an internal-hole saw. As is further disclosed in the above-cited publications, under the aspect of productivity, an especially advantageous application resides in carrying out grinding and sawing processes simultaneously, or with only a slight time difference such that the grinding and sawing processes can be carried out utilizing the same advance motion.
However, this technique illustrated a significant limitation in prior art devices such as those mentioned above. This limitation is that since the slicing process is the more critical of the two processes, parallel operating steps are carried out with an optimum advance motion particular to the slicing process, i.e. that the flow and speed of motion is tuned especially to the slicing process. In the sense of the above stated productivity aspect, the grinding process is carried out utilizing the same advance motion, wherein the process conditions for the grinding process in the general case are not optimal.
In addition, in these prior art devices, the constructional and kinematic boundary conditions require that for the grinding process only plane-side longitudinal grinding is possible wherein the advance speed must be identical with that of the slicing process. These limiting boundary conditions significantly impair the surface quality of the grinding process in a lasting manner. In view of sufficient removal performance and permanent cutability, a relatively coarse grain must be selected for the grinding process on which the surface quality achieved thereby must, by necessity, be relatively coarse. For the further fabrication and additional working of the wafers, it would, however, be particularly advantageous if the plane surface of the wafers generated by grinding would already have a good surface quality that would meet the demands of the subsequent processes without intermediate working.
However, a fundamental problem of grinding technology is encountered. As is known in the art of grinding technology, either grinding takes place with a coarse grain tool and relatively large removal performance with relatively poor surface quality is obtained on the wafer, or grinding takes place with a fine grain tool and low removal performance with good surface quality. In the present case, the high demands of the slicing process and the advance speed connected therewith require a relatively high removal performance and, consequently, a coarse grain which by necessity generates a relatively rough surface on the workpiece. However, as already stated above, a fine surface would be desirable for the further processing and additional working of the wafers.
The standard attempt to provide solution in the art of grinding technology entails a two-stage working: pregrinding with a coarse grain tool and then final-finishing working with a fine grain tool. However, the standard approach used in practice in this simple form is not usable here since two grinding tools in classic form under the given constructional boundary conditions, and utilizing the same advance motion, cannot act upon the same workpiece to thereby subdivide the semiconductor bar.